Commercial process for electroplating nickel-phosphorus coatings

ABSTRACT

An aqueous bath for electroplating NiP coatings is provided comprising, generally, a solution containing nickel salts, either a mixture of nickel sulfate and nickel chloride or an all nickel chloride source, sodium hypophosphite and boric acid. Thiourea is added to an all chloride bath solution for decorative applications or other applications requiring enhanced brightness. Upon dissolution of the bath constituents, the bath pH is adjusted up to a value of 3.5 to 4.5 with sodium hydroxide and maintained for at least 15 minutes at or above room temperature, preferably in the range of 40° to 50° C., then the bath pH is reduced to the bath operating range of 2.0 to 3.0, preferably 2.2 to 2.6. Maintaining the elevated solution pH prior to electroplating a substrate prevents the oxidation of the hypophosphite anions and ensures long-term utilization of the bath.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] Not Applicable

FIELD OF THE INVENTION

[0002] The present invention relates to a bath for electroplating nickel-phosphorus coatings onto substrates and, more specifically, to the pretreatment of the bath for improved bath stability, extended bath life and improved nickel cation-complex formation.

BACKGROUND OF THE INVENTION

[0003] Nickel-phosphorus (NiP) coatings exhibit good corrosion and wear resistance. Many tools are coated with NiP because of the coating's hardness compared to pure nickel coatings. Additional uses are found in catalysis, electrical applications, corrosion protection applications and decorative applications. There are several methods for producing NiP coatings, including rapid quenching of a NiP melt and vapor deposition.

[0004] The most widely used commercial method for plating NiP is through electroless deposition, which uses a hypophosphite anion as a reducing agent in the plating solution. The hypophosphite is reduced to elemental phosphorus during the deposition of nickel and is occluded in the nickel coating. Electroless plating has the distinct advantage of producing coatings of uniform thickness, particularly on substrates having complex shapes. However, electroless plating also has several disadvantages including the following: (a) poor utilization of chemicals used in the bath resulting in higher material costs (5 to 10 times the costs of electrolytic reduction); (b) slow deposition rates (generally less than 25 microns, or approximately 1 mil, per hour); (c) limited coating thickness (generally below 100 microns, or approximately 4 mils) due to roughening; and (d) relatively short bath life requiring treatment and disposal of large volumes of spent bath solution.

[0005] Given the disadvantages of electroless plating, an electrolytic plating process for depositing NiP coatings might be more economical in certain circumstances, such as for plating large quantities of regularly-shaped substrates. However, the primary obstacle to commercializing an electrolytic plating process has been poor bath stability. Instability results in reduced bath life, which in turn increases material costs, increases treatment and disposal costs and generally upsets the electroplating process.

[0006] Electrolytic solutions in the literature generally employ nickel sulfate as the source of nickel cations. Nickel chloride is used to improve anode corrosion, but also increases conductivity and uniformity of the coating thickness distribution. An excessive amount of chloride increases the corrosivity of the solution and the internal stress of the deposit. Conventional baths also contain phosphorus acid, as the source for phosphorus, and phosphoric acid. Boric acid is used to buffer the solution.

[0007] One disadvantage of the conventional bath is that the solubility of nickel phosphite is relatively low at higher solution pH; therefore, the bath is normally operated at a solution pH less than 1.0. At such a low solution pH, the cathode current efficiency is normally less than 50%.

[0008] Another disadvantage of the conventional bath is the tendency to oxidize phosphorus acid to phosphoric acid, which results in shortened bath life and increased material costs. The amount of both acids used in conventional baths is critical. If the concentration of either acid is too high, the deposition of the coating is inhibited. If the concentrations of the two acids are not balanced, the physical properties of the coating are adversely affected.

[0009] U.S. Pat. No. 4,673,468 addressed the tendency to form phosphoric acid and disclosed a method to prevent the oxidation by controlling anode current density, which acts to maintain phosphoric acid concentration at a constant value. However, the current density required to prevent the oxidation can be as high as 500 A/ft² or greater. Use of precious metals anodes (platinum or rhodium) and complex anode construction is required to achieve the high current density. These requirements may restrict wide commercial application.

[0010] Alternative bath solutions have been disclosed that mitigate the disadvantages of using phosphorus acid as the source for phosphorus in NiP coatings. U.S. Pat. No. 5,883,762 discloses the use of sodium hypophosphite as the source for phosphorus. However, sodium hypophosphite used in conventional baths also has disadvantages, namely the thermodynamic instability of the ionic species.

[0011] Sodium hypophosphite forms a complex with the nickel cations present in the bath. The complexing rate is dependent on solution pH. At low pH, the complexing rate is relatively slow; therefore, most of the hypophosphite anions are free in solution and not associated in a complex with a nickel cation. Sodium hypophosphite baths are normally operated at solution pH values of 2.0 to 3.0. Solution pH values above 3.0 do not produce amorphous NiP coatings and significantly reduces cathode current density. Solution pH values of 2.0 to 3.0 are sufficiently low to retard the complexing rate of hypophosphite anions, therefore, free and unstable hypophosphite anions diffuse to and oxidize at the anode to become phosphite anions. While phosphite created by hypophosphite oxidation may also be electrolytically reduced to phosphorus at the cathode, the concentration of phosphite quickly rises during the electroplating process to the point where nickel phosphite precipitates and could be occluded into the coating. The instability of the hypophosphite anions shortens the useful life of the bath and hinders application at an industrial scale.

[0012] The anode does not readily oxidize hypophosphite anions that are complexed with nickel cations; therefore, it is desirable to formulate a bath that decreases the concentration of free, unstable hypophosphite anions by promoting the complexing of the anions with nickel cations.

[0013] With regards to decorative electroplating applications, besides thermodynamically stabilizing the bath, it is also desirable to improve the brightness of the coating's appearance. This can also be accomplished with proper bath formulations.

OBJECTS AND SUMMARY OF THE INVENTION

[0014] It is an object of the invention to provide a thermodynamically stable bath for the electrolytic reduction of NiP coatings.

[0015] It is another object of the invention to extend the useful life of the bath by controlling undesirable reactions that normally take place during the electroplating process.

[0016] It is yet another object of the invention to enhance the formation of nickel ion complexes within the bath prior to the electroplating process.

[0017] Further, it is yet another object of the invention to improve the brightness of NiP coatings by properly formulating the bath.

[0018] These and other objects and advantages of the invention shall become apparent from the following general and preferred description of the invention.

[0019] Accordingly, an aqueous bath for electroplating NiP coatings is provided comprising, generally, a solution containing nickel salts, either a mixture of nickel sulfate and nickel chloride or an all nickel chloride source, sodium hypophosphite and boric acid. Thiourea is added to an all chloride bath solution for decorative applications or other applications requiring enhanced brightness. Upon dissolution of the bath constituents, the bath pH is adjusted up to a value of 3.5 to 4.5 with sodium hydroxide and maintained for at least 15 minutes at or above room temperature, preferably in the range of 40° to 50° C., then the bath pH is reduced to the bath operating range of 2.0 to 3.0, preferably 2.2 to 2.6. Maintaining the elevated solution pH prior to electroplating a substrate prevents the oxidation of the hypophosphite anions and ensures long-term utilization of the bath. At the higher pH, the complexing rate of the hypophosphite anions proceeds at a greater rate. Once complexed, the hypophosphite anions are not readily oxidized by the anode. By maintaining the higher solution pH for at least 15 minutes, the concentration of free, unstable hypophosphite anions is reduced, thereby preventing the oxidation of the unstable anion to more stable and undesirable products.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows the effect of the complexing treatment on the initial precipitation pH value with various current loads passed through the bath per unit of bath solution.

[0021] FIG. 2 shows the effect of varying phosphorus to nickel ratio in the bath on the phosphorus content of the coating.

PREFERRED EMBODIMENTS OF THE INVENTION

[0022] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof. It is to be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention.

[0023] It has been proven by experimental results that initial precipitation pH value of the bath varies with the quantity of electricity passed through the bath per unit bath solution. The initial precipitation pH value of the bath indicates the content of phosphite anions in the bath. A higher initial precipitation pH value corresponds to a lower concentration of phosphite anions. FIG. 1 clearly demonstrates the function of the complexing treatment. The bath that has not undergone the complexing treatment exhibits a rapid drop in initial precipitation pH value from 6.5 to 3.5 within the initial increase of 40A-hr per liter of solution and tends to decrease further. In contrast, the initial precipitation pH value of the treated bath begins to decrease at the onset of electroplating, then ceases and reaches a constant value of approximately 4.5. An initial precipitation pH value of 4.5 is significantly higher than the operating pH value of the hypophosphite bath; hence bright, sound and adhesive NiP coatings are produced in the absence of phosphite anions because formation of phosphite is inhibited. The inhibition of phosphite anion formation significantly extends the useful lifetime of the bath.

[0024] Nickel plate is used as the anode in the present invention. Its current efficiency is above 90%, but below 100% because two anodic reactions compete in this electroplating process, the anodic dissolution of nickel and the oxidation of hypophosphite. The cathode current efficiency of the bath varies from 95 to 105%, while the bath is operated at a solution pH of 2.2 to 2.6 and a temperature of 65 to 75° C. without air agitation. The cathode current efficiency decreases to about 90% when air agitation is employed. The fact that the cathode current efficiency may exceed 100% implies that reduction of nickel may be occurring by chemical reduction (electroless) to some extent concurrently with the electrolytic reduction. Electroplating experiments over the course of 48 hours have shown that the nickel concentration in the bath remains relatively constant when air agitation is not employed, however, the nickel concentration slowly rises if air agitation is employed.

[0025] The phosphorus content of the NiP coating closely relates with bath pH value and the content ratio of phosphorus to nickel of the bath. Electroplating experiments utilizing the disclosed bath and complexing treatment show that the phosphorus content of the NiP coating initially rises with an increase of phosphorus to nickel ratio maintained in the bath. However, the relationship is not linear. Eventually, the phosphorus content of the coating reaches a value after which further increases in phosphorus to nickel ratio does not result in an appreciable increase in the phosphorus content. This relationship is depicted in FIG. 2. The phosphorus content of the coating produced from the disclosed bath operating at a solution pH of 2.2-2.6, a temperature of 65-75° C. and cathode current density of 2-4A/dm² without air agitation is in the range of 8-9 wt. %. The phosphorus content of the coating increases to 11-12 wt. % if air agitation is employed.

[0026] Electroplating applications utilizing the disclosed bath should be pH controlled. The pH of the bath should be controlled within the range of 2.0 to 3.0, but preferably in the range of 2.2 to 2.6. If the pH of the bath decreases below 2.0, the stability of the bath may possibly be destroyed.

[0027] The temperature of the disclosed bath should also be controlled by conventional means. While electroplating will occur at room temperature, plating rate and current efficiency both increase with higher bath temperatures. Furthermore, NiP coatings exhibit high internal stress when plated at room temperature. Bright, low stressed coatings can be produced with acceptable rates of deposition at bath temperatures greater than 60° C. The preferable operating temperature range of the bath is 65 to 75° C.

[0028] One specific embodiment of the disclosed bath comprises an aqueous mixture of nickel sulfate and nickel chloride, sodium hypophosphite and boric acid. The concentrations of the various constituents are as follows: Nickel sulfate NiSO₄.6H₂O 150-200 g/L Nickel chloride NiCl₂.6H₂O 45 g/L Sodium hypophosphite NaH₂PO₂.H₂O 10-50 g/L Boric acid H₃BO₃ 20-40 g/L

[0029] Another specific embodiment of the disclosed bath comprises an aqueous mixture of nickel chloride, sodium hypophosphite and boric acid. The concentrations of the various constituents are as follows: Nickel chloride NiCl₂.6H₂O 150-250 g/L Sodium hypophosphite NaH₂PO₂.H₂O 10-60 g/L Boric acid H₃BO₃ 20-40 g/L

[0030] Another specific embodiment of the disclosed bath comprises an aqueous mixture of nickel chloride, sodium hypophosphite, boric acid and thiourea. The concentrations of the various constituents are as follows: Nickel chloride NiCl₂.6H₂O 150-250 g/L Sodium hypophosphite NaH₂PO₂.H₂O 10-60 g/L Boric acid H₃BO₃ 20-40 g/L Thiourea NH₂CSNH₂ 0.05-0.2 g/L

[0031] Experimental results show that all three embodiments are similar in many respects. One difference, however, is that the internal stress of the coating produced from an all chloride bath is much lower than that of the coating produced from a bath based on the nickel sulfate and nickel chloride combination. The limiting current density of the bath based on the nickel sulfate and nickel chloride combination operating at solution pH of 2.2 to 2.6, a bath temperature of 65 to 75° C. and without air agitation is approximately 1.5A/dm². The limiting current density of the same bath operating with air agitation is 2.0-2.5 A/dm². Exceeding these limiting current densities may result in cracking and flaking of the coating due to high internal tensile stress. Therefore, the maximum plating rate corresponding to the limiting current densities is approximately 32 microns per hour if air agitation is employed and even less if air agitation is not employed. In contrast, the limiting current density of the bath based only on nickel chloride operating at the same solution pH and bath temperature without air agitation is 3.0-3.5 A/dm² and 4.0-4.5A/dm² with air agitation. Correspondingly, the maximum plating rates of an all chloride bath operating with and without air agitation are approximately 55 microns per hour and 42 microns per hour, respectively. Moreover, the all chloride bath has a higher conductivity and greater covering power compared with the combination bath. Therefore, it is preferable to use the all chloride bath.

[0032] With respect to decorative applications, the brightness of the NiP coating plated from an all chloride bath is significantly enhanced with the addition of thiourea to the bath formulation.

[0033] Bright, sound and adhesive NiP coatings containing various concentrations of phosphorus can be produced from any of the disclosed baths. Varying the phosphorus to nickel ratio of the bath controls the phosphorus content of the coating. An amorphous NiP coating containing greater than 8 wt % P can be deposited as long as the phosphorus to nickel ratio of the bath exceeds 0.25. NiP coatings containing more than 10 wt % P can only be produced with air agitation. Other forms of agitation, such as mechanical agitation or other type of agitation can also be used as long as the agitation is vigorous and uniform over the entire surface of the substrate to be plated.

[0034] Before preparing the bath the required concentration of sodium hypophosphite should be calculated. After all of the constituents of the bath are dissolved, the bath pH is adjusted above 3.5, preferably in the range of 3.8 to 4.2, with sodium hydroxide. The bath pH should be maintained in this range for at least 15 minutes. The bath can be maintained at room temperature, but an elevated temperature obtains better results. The optimum temperature is in the range of 40° C. to 50° C. Temperatures above 50° C. may cause the chemical reduction of the hypophosphite-nickel complex resulting in the undesirable formation of phosphite. The bath pH is then reduced to its preferred operating range of 2.2-2.6 with hydrochloric acid or sulfuric acid according to bath formulation. Once the pH is reduced, the bath solution is ready for electroplating.

[0035] In practice, the cathode current density will vary, depending upon the particular geometry of the cathode substrate and other variables. Electroplating with a bath based on nickel sulfate and nickel chloride, the normal operating current density is 1-2 A/dm² and the corresponding plating rate is 13 microns to 26 microns per hour. With an all chloride bath, the normal operating current density is 3-4 A/dm² and the corresponding plating rate is 39 microns per hour to 52 microns per hour.

[0036] During the electroplating process, sodium hypophosphite is added to the bath to replenish the phosphorus concentration depleted by deposition of the coating in order to maintain the phosphorus to nickel ratio of the bath. The amount of sodium hypophosphite added is dependent on the quantity of electricity passed through the bath. When the bath is operated without air agitation, 300 grams of sodium hypophosphite per kiloampere-hour should be added to the solution. When the bath is operated with air agitation, 400 grams of sodium hypophosphite per kiloampere-hour should be added. In order to maintain bath stability, each addition of sodium hypophosphite must also be limited to a concentration of 1-2 grams per liter of bath solution. Accurate control of the sodium hyophosphite concentration necessitates analysis of phosphorus and nickel concentrations during the electroplating process.

[0037] Bath pH may slowly rise, particularly when the bath it is operated without air agitation. Bath pH should be monitored and controlled by the addition of hydrochloric acid or sulfuric acid according to the bath formulation.

[0038] The initial precipitation pH value of the bath may be used as a very convenient and useful means to determine the phosphite concentration of the bath. Normally, the initial precipitation pH value is approximately 4.5.

[0039] Additionally, the bath solution should be continuously filtered. Solid particles can be occluded into coating and roughen the coating surface. Excessive particles in the bath may also destroy the bath stability due to the concurrent chemical reduction of nickel.

[0040] The disclosed baths can be used for applying NiP coatings onto many different substrates, such as steels, copper and copper alloys, aluminum and aluminum alloys, and others. Before electroplating the substrates, they must first be properly pretreated by conventional methods and procedures. Examples of conventional pretreatment methods include: ASTM B320, Standard Practice for Preparation of Iron Castings for Electroplating; ASTM B242, Standard Practice for Preparation of High-Carbon Steels for Electroplating; ASTM B183, Standard Practice for Preparation of Low-Carbon Steels for Electroplating; ASTM B253, Standard Guide for Aluminum Alloys for Electroplating; A. E. Wyszynski, “An Immersion Alloy Pretreatment For Electroplating On Aluminum,” Transactions of the Institute of Metal Finishing, 1967, Vol. 45; and H. Silman, et al., “Protective Coatings for Metals.” Finishing Publications LTD, 1978. After pretreatment the substrates, they are immersed into the bath solution and connected to the negative terminal of a D.C. power supply.

[0041] The disclosed baths are illustrated further by means of examples. All of the bath solutions used in the following examples were complex treated by the method disclosed above prior to electroplating. Unless otherwise indicated in the examples, all coatings were electroplated onto carbon steel panels (1.0×0.5 dm) using nickel plate as the anode.

EXAMPLE 1

[0042] Nickel Sulfate NiSO₄.6H₂O 150 g/litre Nickel Chloride NiCl₂.6H₂O 45 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 50 g/litre Boric acid H₃BO₃ 30 g/litre Content ratio R/Ni 0.33 Temperature 70 ± 2° C. pH 2.2-2.5 Cathode current density 2.0 A/dm² Plating suration 120 minutes

[0043] A bright and adhesive coating was obtained with current efficiency of 89%. The measured hardness of the coating was Hv 620.

EXAMPLE 2

[0044] Nickel Chloride NiCl₂.6H₂O 200 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 50 g/litre Boric acid H₃BO₃ 30 g/litre Content ratio P/Ni 0.30 Temperature 70 ± 2° C. pH 2.3-2.5 Cathode current density 3.0 A/dm² Plating duration 60 minutes

[0045] A silvery-white coating having an average thickness of 39 microns was electroplated with current efficiency of 103% under the above conditions from a bath having the above composition.

EXAMPLE 3

[0046] Nickel Chloride NiCl₂.6H₂O 180 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 20 g/litre Boric acid H₃BO₃ 30 g/litre Content ratio P/Ni 0.13 Temperature 70 ± 2° C. pH 2.2-2.5 Cathode current density 3.0 A/dm² Plating duration 60 minutes

[0047] A silvery-white coating containing 6.8 wt % phosphorus was electroplated with a current efficiency of 98% under the above conditions from a bath having the above composition.

EXAMPLE 4

[0048] Nickel Chloride NiCl₂.6H₂O 180 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 50 g/litre Boric acid H₃BO₃ 30 g/litre Content ratio P/Ni 0.33 Temperature 70 ± 2° C. pH 2.3-2.5 Cathode current density 4.0 A/dm² Plating duration 60 minutes

[0049] An bright coating was obtained with current efficiency of 91% under the above conditions from a bath having the above composition. The coating contained 11.6 wt % phosphorus. The measured hardness was Hv 645.

EXAMPLE 5

[0050] Nickel Chloride NiCl₂.6H₂O 200 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 50 g/litre Boric acid H₃BO₃ 30 g/litre Content ratio P/Ni 0.30 Temperature 60 ± 2° C. PH 2.3-2.5 Cathode current density 2.5 A/dm² Plating duration 60 minutes

[0051] The coating produced at 60° C. had higher internal stress and exhibited cracks on the edge of the panel. The current efficiency of electroplating was 91%. The coating consisted of 9.0 wt % phosphorus and 91 wt % nickel.

EXAMPLE 6

[0052] Nickel Chloride NiCl₂.6H₂O 180 g/litre Sodium hypophosphite NaH₂PO₂.H₂O 40 g/litre Boric acid H₃BO₃ 30 g/litre Thiourea NH₂CSNH₂ 0.1 g/litre Content ratio P/Ni 0.263 Temperature 70 ± 2° C. PH 2.2-2.5 Cathode current density 3.0 A/dm² Plating duration 60 minutes

[0053] Mirror-like bright coating was obtained with current efficiency of 99%. The coating contained 8.4% nickel and 0.93% sulfur.

EXAMPLE 7

[0054] Example 4 was repeated except that an aluminum panel having the same area as the carbon steel panel was coated. Prior to electroplating the NiP coating, the aluminum substrate was cleaned with a mild alkaline cleaner then treated with a double immersion zincate process. Finally, the aluminum panel was electroplated in a bronze electrolyte for 3 minutes. A bright and adhesive nickel phosphorus coating was produced on the prepared aluminum panel.

EXAMPLE 8

[0055] A two-liter bath was utilized to coat carbon steel shafts. The shafts were 12 mm in diameter and 135 mm in length, corresponding to an area of approximately 0.5 dm² per shaft. The initial bath solution comprising 200 g/L of nickel chloride, 50 g/L of sodium hypophosphite and 30 g/L of boric acid was prepared and complex treated. Two nickel plates were employed as anodes and were placed on both sides of the bath. Three shafts were degreased in hot alkaline cleaning solution, dipped in 1:1 hydrochloric acid and then placed in the center of the bath and connected to the negative terminal of a D.C. power supply. The bath was operated at a temperature of 70±2° C., a bath pH in the range of 2.2 to 2.6 and without air agitation. 4.5 A was supplied from a D.C. rectifier and maintained accurately to keep the cathode current density at 3.0 A/dm². The three shafts were electroplated with NiP coatings for two hours, after which they were removed and replaced by three new shafts similarly prepared. Sets of three shafts were continuously electroplated in two-hour intervals over a 48-hour period. 2.7 grams (0.3 grams/A.H.) of sodium hypophosphite was added to the bath at intervals to replenish the phosphorus. A bright, sound and adhesive NiP coating containing 8 to 9 wt % P was consistently obtained. The concentration of nickel in the bath was analyzed before and after the test and remained relatively constant. After the test, the initial precipitation pH value of the bath was measured to be 4.5.

[0056] Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention. 

What is claimed is:
 1. A composition of matter for producing electrodeposited nickel-phosphorus coatings, the composition being an aqueous electroplating bath comprising: a source of nickel selected from the group consisting of nickel sulfate and nickel chloride; sodium hypophosphite; boric acid; and thiourea. 